With the continuing demand for electronic parts to reduce size and overall profile and improve performance, active efforts are made to develop low-cost high-density packaging technology. For the connection of such microcircuits, conventional solders and rubber connectors are difficult to manage, and instead, anisotropic conductive materials having a good resolving power are often used. With this method, fine electrodes on a liquid crystal display (LCD) module and fine electrodes on a tape automated bonding (TAB) or flexible printed circuit (FPC) substrate are connected by adding an amount of a fine conductive filler to an adhesive resin to form an anisotropic conductive resin composition, interposing the composition between the opposed electrodes and heating under pressure for collectively forming connections between a plurality of electrodes.
From the past, the conductive fillers used in this technology are metal fine powders and organic or inorganic particles which have been metallized by electroplating or the like. The metal fine powders generally have a broad particle size distribution and a problem arises with respect to connection reliability due to influences of conduction, insulation, connection mechanism and the like. As for the conductive particles in the form of metallized plastic particles, conductive particles having a narrow particle size distribution are available, but tend to agglomerate together, offering a reduced contact area and a reduced retaining force for connection between electrodes. It is thus quite difficult to handle microminiaturization by adding conductive particles in a small amount sufficient to avoid agglomeration.
On the other hand, liquid crystal display modules make progress toward further miniaturization and higher reliability. It is thus desired to reduce the conventional electrode pitch of about 200 μm (5 electrodes/mm) to a pitch of up to 100 μm (at least 10 electrodes/mm) while a further resolution improvement will be required in the future. While a rapid progress is made to reduce the size and weight of electronic parts, great efforts are made to utilize anisotropic conductive materials in a packaging technique of directly mounting a bare chip onto a glass substrate, generally known as chip-on-glass (COG), or a packaging technique of directly mounting a bare chip onto a printed board, generally known as chip-on-board (COB). In concert with these efforts, conductive fine particles added to the anisotropic conductive materials are desired to improve the properties that account for the reliability of connection between electrodes.
JP-A 51-135938, JP-A 5-21094, JP-A 8-7658, and JP-A 10-184962 disclose anisotropic conductive adhesive films with high connection reliability using conductive particles obtained by metallizing polymer particles. These adhesive films, however, may fail to achieve satisfactory conduction and bond strength at the same time for connections at fine pitches between opposed circuits.
For the packaging of bare chips using anisotropic conductive resins, a variety of performance factors are required. One factor is connection reliability in fine-circuitry connection. To increase the resolution of the prior art anisotropic conductive resins, engineers attempted to ensure insulation between opposed circuits by using conductive fine particles with a particle size smaller than the insulating portion between the circuits, and adjusting the amount of conductive fine particles added so as to avoid any contact between conductive particles, thereby ensuring conduction to the circuit connections. Regrettably, reducing the particle size of conductive fine particles means that the number of particles and the surface area thereof are substantially increased so that particles undergo secondary agglomeration, with a failure to ensure insulation between opposed circuits. Also, reducing the addition amount of conductive particles means that the number of conductive particles on a circuit to be connected is reduced, resulting in a smaller number of contact points, with a failure to ensure conduction to the circuit connection. It is thus very difficult to increase resolution while maintaining connection reliability.
Another factor is how to overcome the instability of connection due to variations in height of electrodes on a bare chip or IC and variations in height on glass substrate or printed board side. It would be desirable if conductive fine particles accommodate such variations in height to provide stable connections to all terminals.